The arrests were made during 'initial inquiries' of investigations launched in May into how horsemeat ended up in food products such as beef burgers. Photograph: Niall Carson/PA

Two men have been arrested on suspicion of conspiracy to defraud and two others interviewed under caution by police as international investigations continue into how food became adulterated by horsemeat.

A number of UK homes and businesses have also been searched by the City of London force which leads nationally on fraud. The arrests were made during "initial inquiries" after investigations were launched in May but only announced on Thursday evening.

Detective Chief Superintendent Oliver Shaw said: "This is an extremely complex investigation covering a number of jurisdictions and a variety of businesses. We are working closely with police forces, other law enforcement agencies and regulators to determine whether horse meat being used in a range of meat products was deliberate and coordinated criminal activity."

Last month, the Commons environment, food and rural affairs committee, criticised the slow pace of UK and Irish authorities blaming them for failing to acknowledge the extent of the network of companies trading in mislabelled meat or prosecute any firm involved for the scandal which first became public in January.

City of London police said they had initially been approached to review all the available evidence collected by law enforcement agencies in the UK and Europe, as well as from the Food Standards Agency's "regulatory activity" taking place in different parts of the country.

The Commons committee said: "The evidence we received from retailers and food processors in the UK and Ireland suggests a complex, highly organised network of companies trading in and mislabelling frozen and processed meat or meat products in a way that fails to meet specifications and that is fraudulent and illegal.

"We are concerned at the failure of authorities in both the UK and Ireland to acknowledge the extent of this and to bring prosecutions.

"We are dismayed at the slow pace of investigations and would like assurance that prosecutions will be mounted where there is evidence of fraud or other illegal activity."

The FSA has already agreed to an independent review of its response to the scandal. Horse DNA was first discovered in processed beef products in Ireland last December.